1. Field of the Invention
The present invention relates to an alloy type thermal fuse in which a Bi—Sn alloy is used as a fuse element, and which has an operating temperature of about 140° C., and also to a material for a thermal fuse element.
An alloy type thermal fuse is widely used as a thermo-protector for an electrical appliance or a circuit element, for example, a semiconductor device, a capacitor, or a resistor.
Such an alloy type thermal fuse has a configuration in which an alloy of a predetermined melting point is used as a fuse element, the fuse element is bonded between a pair of lead conductors, a flux is applied to the fuse element, and the flux-applied fuse element is sealed by an insulator.
The alloy type thermal fuse has the following operation mechanism.
The alloy type thermal fuse is disposed so as to thermally contact an electrical appliance or a circuit element which is to be protected. When the electrical appliance or the circuit element is caused to generate heat by any abnormality, the fuse element alloy of the thermal fuse is melted by the generated heat, and the molten alloy is divided and spheroidized because of the wettability with respect to the lead conductors or electrodes under the coexistence with the activated flux that has already melted. The power supply is finally interrupted as a result of advancement of the spheroid division. The temperature of the appliance is lowered by the power supply interruption, and the divided molten alloys are solidified, whereby the non-return cut-off operation is completed.
Conventionally, a technique in which an alloy composition having a narrow solid-liquid coexisting region between the solidus and liquidus temperatures, and ideally a eutectic composition is used as such a fuse element is usually employed, so that the fuse element is fused off at approximately the liquidus temperature (in a eutectic composition, the solidus temperature is equal to the liquidus temperature). In a fuse element having an alloy composition in which a solid-liquid coexisting region exists, namely, there is the possibility that the fuse element is fused off at an uncertain temperature in the solid-liquid coexisting region. When an alloy composition has a wide solid-liquid coexisting region, the uncertain temperature width in which a fuse element is fused off in the solid-liquid coexisting region is correspondingly increased, and the operating temperature is largely dispersed. In order to reduce the dispersion, therefore, an alloy composition having a narrow solid-liquid coexisting region between the solidus and liquidus temperatures, or ideally a eutectic composition is used.
Because of increased awareness of environment conservation, the trend to prohibit the use of materials harmful to a living body is recently growing as a requirement on an alloy type thermal fuse. Also an element for such a thermal fuse is strongly requested not to contain a harmful element (Pb, Cd, Hg, Tl, etc.).
Conventionally, a Bi—Sn eutectic alloy (57% Bi, balance Sn) is known as an element for a thermal fuse which does not contain an element harmful to a living body, and which has an operating temperature of about 140° C.
2. Description of the Prior Art
Conventionally, functions of an electrical appliance are advanced, and the power consumption of an appliance is increased. Therefore, a thermal fuse is requested to have a high power rating of AC 250 V and 5 A or more.
When an alloy type thermal fuse is used at a voltage as high as AC 250 V, an arc is easily generated at an operation of the fuse. As a result, substances such as a charred flux produced by the arc, and molten portions of a fuse element are scattered to adhere to the inner wall of a case, thereby forming a resistor path, and a current may flow through the resistor path. The thermal fuse may be damaged or broken by Joule's heat due to the current. In succession to the current flow through the resistor path, or after interruption of the current flow, a rearc may be generated, and the thermal fuse may be damaged or broken by the rearc. Even when the thermal fuse may not be damaged or broken, the insulation property after an operation is lowered to produce the probability that, when a high voltage is applied, reconduction occurs to cause a serious problem.
The degrees of the damage or destruction modes of a thermal fuse depend on the level of the destruction energy. The modes are enumerated in the order of degree as follows: ejection of a molten fuse element or a molten flux; destruction of a sealing portion; destruction of an insulating case; and melting of a lead conductor or an insulating case.
When a thermal fuse in which the above-mentioned Bi—Sn alloy is employed as a fuse element is used under a high voltage, an abnormal mode such as damage or destruction at an operation or an insulation failure after an operation easily occurs. The reason of this is estimated as follows. At an operation, a fuse element is changed at once from the solid phase to the liquid phase in which the surface tension is low, without substantially entering an intermediate phase state. When the fuse element is fused off, therefore, the liquefied fuse element is formed into minute particles, and the particles are scattered together with a charred flux due to an arc at the operation. Many of the particles adhere to the inner wall of an outer case, thereby causing the insulation distance after an operation not to be maintained. As a result, such an abnormal mode is caused by the reconduction due to the high-voltage application or generation of a rearc after reinterruption.
The inventor eagerly conducted studies in order to prevent an abnormal mode from occurring when a thermal fuse in which a Bi—Sn alloy is used as a fuse element operates. As a result, it has been found that, when a composition of Bi of larger than 50% and 56% or smaller, and the balance Sn is employed, an abnormal mode can be satisfactorily prevented from occurring and dispersion of the operating temperature can be sufficiently reduced.
The reason why an abnormal mode can be prevented from occurring is estimated as follows. In the specific Bi—Sn alloy composition, a solid-liquid coexisting region (intermediate state) in which the surface tension is relatively large exists with being deviated from a eutectic point and between the solidus temperature and the liquidus temperature. The spheroid division of the fuse element is caused in the intermediate state. As a result, scattering in the form of minute particles hardly occurs. The reason why, contrary to the above-mentioned usual technique, dispersion of the operating temperature of a thermal fuse can be suppressed to a low level even in an alloy composition of a wide solid-liquid coexisting region is estimated as follows. Referring to DSC measurement results shown in FIGS. 8 to 10, the surface tension of a state in the vicinity of the peak p that is the terminal of a process in which a change from the solid phase to the liquid phase rapidly advances reaches a low one necessary for the spheroid division of the fuse element, even before the liquidification process reaches the end (the liquidus temperature).